Hands down. You are right. That tank is in serious trouble. He is mistaking overstocking for overcrowding. The two are different things. Overstocking refers to the amount of waste the bio-bugs in the tank can handle. Overcrowding refers to the amount of swimming space for the fish. The tank may not be overcrowded, but it is overstocked.
I've seen people use the "we've been doing this a long time" excuse/defense a lot of times. 9/10 they are have been "doing it" wrong. (lol read the Old Pro [bad] at the bottom of this article What kind of fishkeeper are you?
The research is out there if he wants to learn.
- brilliant loach site. I'm a bit of a loach fanatic, so I spend a lot of time here.
- wonderful articles on a lot of uncommon (and common) fish in the hobby
TFK: Tropical Fish Profiles
- great articles on most of the common fish and plants in the hobby
- UK-based fishkeeping magazine. It's the best in the world.
Tropical Fish Hobbyist Magazine
- second best fishkeeping magazine, US-based
Aquarium Fish International
- another fishkeeping website, US-based
Tell him to compare the advice of these sites to what he's getting at the pet store.